Method of forming corrosion resistant films on steel plates

ABSTRACT

A PROCESS FOR PRODUCING A SURFACE-TREATED STEEL PLATE HIGH IN THE ANTICORROSIVENESS, PARTICULARLY ADAPTED AS A MATERIAL FOR MAKING CANS, ON THE PRINCIPLE OF THE THERMODECOMPOSING PLATING, WHEREIN THE STEEL PLATE IS COATED WITH AN AQUEOUS SOLUTION CONTAINING NITRATE AND/OR ACETATE OF NI AS THE MAIN COMPONENT THEREOF AND OF OTHER METALS SUCH AS CR, MN, ZN AND AL AS SELECTIVE COMPONENTS AND THEN HEATED AT A CERTAIN RANGE OF TEMPERATURE SO AS TO CAUSE THE THERMODECOMPOSING REACTION TO CAUSE THEREBY A STRONG FILM CONTAINING METALLIC NICKEL AND AT LEAST ONE METAL OXIDE TO FORM ON THE SURFACE OF THE STEEL PLATE.

United States Patent O 3,677,797 METHOD OF FORMING CORROSION RESISTANTFILMS ON STEEL PLATES Hidejiro Asano and Yashichi Ouyagi, Kitakyusliu,Japan, assignors to Nippon Steel Corporation, Tokyo, Japan No Drawing.Filed Apr. 27, 1970, Ser. No. 32,439 Int. Cl. C23c 3/04; C23f 17/00 US.Cl. 117-71 M 15 Claims ABSTRACT OF THE DISCLOSURE A process forproducing a surface-treated steel plate high in the anticorrosiveness,particularly adapted as a material for making cans, on the principle ofthe thermodecomposing plating, wherein the steel plate is coated with anaqueous solution containing nitrate and/or acetate of Ni as the maincomponent thereof and of other metals such as Cr, Mn, Zn and Al asselective components and then heated at a certain range of temperatureso as to cause the thermodecomposing reaction to cause thereby a strongfilm containing metallic nickel and at least one metal oxide to form onthe surface of the steel plate.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to a method of making surfacetreated steel plates bythermodecomposing plating.

Description of the prior art As well known, as a material for makingcans, a tinplated steel plate is used most today. However, tin has suchproblems that it is expensive, is not endurable to the use at a hightemperature, is weak against corrosion in the atmosphere and furtherbecomes black depending on the contents in case it is made a can.Furthermore, tin is so short as a resource in the world that its supplyis unstable. As a result in the world that its support is unstable. As aresult of making researches on surface-treated steel plates the presentinventors have discovered that in order to produce surface-treated steelplates having no such defects as in the case of the above-mentionedtin-plated steel plate it is necessary to have a film, which is morestable than the steel plate to be plated and is to be formed on atechnical idea different from the plating, such as tin-plating, forelectrochemically protecting the steel plate in a corrosive liquid, onthe surface of the steel plate.

Further, when using metal-plated surface-treated steel plates, includinga tin-plated steel plate, as a material for making cans, it is usual tocoat the metal-plating layer with well known lacquers of various kindsas, for example, an epoxy resin series paint for the necessity toprevent the metal-plating layer from being corroded and the metal ionsfrom being melted-out by such contents in the can as, for example, abeverage, liquor, oil or fat.

Therefore, as for a metal-plated surface-treated steel plate thefollowing conditions are required that not only it should be high in theantirusting property in the atmosphere but also, in case it is coatedwith a lacquer and is then dipped in the above described corrosiveliquid, the lacquer coating film should not peel off, and moreover itshould not interfere with the can-manufacturing operations, forinstance, it should be easy to solder and mold.

As regards the peeling-off of such lacquer film, particularly the caseof coating an Alor Zn-plated steel plate with a lacquer can be taken asan example. When dipping said steel sheet in the above describedcorrosive liquid, the corrosion of the Alor Zn-plating layer by saidcorrosive liquid proceeds faster than the corrosion of the iron ice basematerial on account of an anodic protective action of Al or Zn on thesteel sheet, resulting in the peeling-01f the lacquer film.

Different from the case of Alor Zn-plated steel plate there occurs nopeeling-off of a lacquer when applied on the surface of a Cror Ni-platedsteel plate, because Cr or Ni is inherently high in the antirustingproperty and is more stable than the iron base when dipped in suchcorrosive liquid as above-mentioned, consequently the lacquer adheresfirmly to the surface of the Cror Ni-plated steel plate. However,because the plating layer is thick, this plating method is not economic.

SUMMARY OF THE INVENTION On the basis of such knowledge as is describedabove, the present inventors have developed a novel process forobtaining a surface-treated steel plate high in the anticorrosion byforming on a steel plate a film more stable than the iron base.

An object of the present invention is to provide surfacetreated steelplates which are so high in the anticorrosiveness that they .cansufiiciently serve in place of tin-plated steel plates, particularlynovel and economic steel plates to be used as material for manufacturingcans, generally called as tin-free steel plate and other steel plates tobe used as materials for automobiles, constructions and toys.

In order to attain the object as above-mentioned the present inventionis to provide methods which are characterized by the following features:that is, a method for obtaining surface-treated steel plate having anexcellent anticorrosiveness wherein an aqueous solution of nitrateand/or acetate of Ni is applied on the surface of a steel platepreviously subjected to a surface-cleaning treatment and is then heatedin a nonoxidative gas atmosphere so as to cause the thermodecomposingreaction to thereby form a film containing metallic nickel on thesurface of the steel plate, and a method for forming a further improvedfilm, that is, a method for obtaining surface-treated steel plate havingan excellent anticorrosiveness wherein an aqueous solution of nitrateand/or acetate of Ni with the addition of one or more of nitrates andacetates of Cr, Mn and Zn is applied on the surface of a steel platepreviously subjected to a surface-cleaning treatment and is then heatedin a nonoxidative gas atmosphere so as to cause the thermodecomposingreaction to thereby form a film containing metallic nickel and one ormore of oxides of Cr, Mn and Zn, and further a method for forming a moreexcellent film, that is, a method for obtaining surface-treated steelplate having a particularly excellent anticorrosiveness wherein anaqueous solution of a nitrate and/or acetate of Ni with the addition ofone or more of nitrates and acetates of Cr, Mn and Zn and with a furtheraddition of a nitrate and/or acetate of Al is applied on the surface ofa steel plate previously subjected to a surface-cleaning treatment andis then heated in a nonoxidative gas atmosphere so as to cause thethermodecomposing reaction to thereby form a film containing metallicnickel and one or more of oxides of Cr, Mn, Zn and Al in additionthereto.

The present invention is particularly very economical because it ispossible to utilize a heating cycle of annealing conditions for thethermodecomposing reaction, and the method of the present invention istherefore very high in practical value.

Moreover, the thus obtained treated film is not only excellent in theanticorrosive activity but is also very ex.- cellent in the paintadhesiveness and mechanical workability. Therefore, the presentinvention is particularly adapted as a method of making steel plates formaking cans.

The method of the present invention shall be detailed in the following.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method which makes thefoundation of the present invention is to form a very thin film ofmetallic nickel on the surface of a steel plate by applying an aqueoussolution, for example, of nickel nitrate to coat the said surface of thesteel plate and heating it in a reductive or inert nonoxidativeatmosphere so as to cause the thermodecomposing reaction of the treatingsolution and the reduction (complete or partial) of nickel nitrate tometallic nickel.

Nickel nitrate or nickel acetate which is an essential component of thetreating aqueous solution of the present invention is used alone or asmixed. However, any other Ni compound (particularly oxide) than this hasa difficulty in the anticorrosiveness, paint adhesiveness andworkability of the treated film and therefore should be avoided in themethod of the present invention.

In the following, for the convenience of the explanation, the aqueoussolution containing an Ni-salt prepared as described above shall becalled a treating solution. Depending on the kinds, some of suchtreating solutions are difficult to uniformly apply to coat steelplates. However, in such case, an improvement can be obtained by addinga proper amount of such surface active agent as, for example, a nonionicactive agent. The deposited amount of the treating solution is soremarkably different depending on the method of coating the surface ofthe steel plate such as blowing a spray, applying with a roller ordipping the steel plate in the solution that the concentration of thetreating solution is difiicult to uniformly define but, if the effectivequantitative range is to be defined in relation to the viscosity of thesolution or the uniformity of the decomposing reaction, Ni ions are tobe contained in an amount of 0.5 to 1 g./l. or preferably 1 to 20 g./l.,later described Cr+++ ions are less than 20 g./l. or preferably g./l.,Al ions are less than g./l. or preferably less than 10 g./l., Zn ionsare less than 40 g./l. or preferably less than 10 g./l. and Mn++ ionsare less than 20 g./l. or preferably less than 10 g./l.

Further, in order to improve the characteristics of the treated film, anitrate or acetate of Mg, Ca or K may be added in an amount of about 1g./l.

It is wise to prevent as much as possible the pH of the treatingsolution of the present invention from becoming excessively acidic toavoid an exchanging reaction with Fe. Therefore, in the industrialpractice, it is desirable to make the pH of the treating solution 3 to4.

As to a film which is heavily colored in its appearance afterheat-treated, there is recognized a tendency of the lacquer adhesivenessbeing reduced and also the adhesion of the film itself to the base beingreduced. Therefore, it is desirable to control the deposited amount ofthe film so that the appearance after the heat-treatment may havetransparency of such a degree as will not substantially impair theluster of the cold-rolled steel plate or may have light tone of such adegree as can be barely recognized with the naked eye (that is, themetal thickness may be less than In or, if possible, about 0.1 or less).

Now, in the method of the present invention, the above describedtreating solution is applied and is then quickly heated to cause athermodecomposing reaction at a temperature of 200 to 750 C. in afurnace of a gas atmosphere of such nonoxidative gas in which, forexample, the H content is 2 to 20% and the main component of the rest isN as DX or NX gas used as a brightly annealing gas so that a strongmetallic nickel film may be formed. When the content of H in the gasatmosphere is less than 2%, the Ni salt is so hard to reduce that thereis formed an undesirable film, because, the film having metallic nickelbecomes lusterless and blackish. On the other hand, the content of H ofmore than 20% is a great loss to the economy. In addition thereto such ahigh content of H is not desirable, particularly in the method, whereinsalts of Al, Cr, Mn and Zn are added to a salt of Ni, because thereduction of salts of these metals proceeds too far with a result ofthat such a metallic nickel-plated film containing oxides of thesemetals as is intended in the present invention can not be obtained.

The above described nonoxidative gas may contain a slight amount ofcarbon dioxide, carbon monoxide or water. However, the heatingatmosphere must not be oxidative by containing oxygen in such a degreeas will have a bad influence on the reduction of the salt of nickel tometallic nickel, because otherwise the salt of nickel applied on thesurface of a steel plate may not be reduced to metallic nickel of arequired amount within the time of the annealing condition and mayremain as nickel oxide on the surface of the steel plate, which does notsuit with the object of the present invention.

When the heating temperature is less than 200 C., no effectivethermodecomposition takes place. On the contrary, when it is more than750 C., there are produced undesirable results that all the metallicnickel produced by the thermodecomposition will alloy with the iron baseto form a hard film which is not only low in the adhesiveness but alsolow in the anticorrosiveness, and further even mechanical properties ofthe steel plate itself, which is to be treated, are often impairedthereby.

In the foregoing an example of applying a treating solution composed ofan Ni-salt only has been explained. The surface film obtained by thismethod is thought to be a two-layer film high in the anticorrosiveness,consisting of an Fe-Ni alloy in the lower layer and metallic nickel inthe upper layer. As a result of making further investigations, however,the present inventors have succeeded in developing an improved method offorming on the surface of a steel plate a film which is more excellentthan the above described film. This film is thought to be formed of twolayers, wherein the lower layer is made of an Fe-Ni alloy and the upperlayer is a mixture of oxides of Cr, Mn and Zn and metallic nickel, ofwhich the mixture, however, is mainly composed.

That is, the improved method is that, wherein an aqueous solution (whichshall be called a treating solution with additive for the convenience ofthe explanation hereinafter) obtained by adding one or more of nitrateand acetates of Cr, Mn arid Zn to a nitrate and/ or acetate of Ni isapplied to coat the surface of a steel plate and is heated in anon-oxidative gas atmosphere furnace to form a film consisting of one ormore of oxides of Cr, Mn and Zn and metallic nickel. The above-mentionedtreating solution with additive may be prepared by adding one or more ofcarbonates, oxalates and hydroxides of Cr, Mn and Zn to an aqueoussolution of Ni nitrate and/or acetate.

In the foregoing, there have been shown examples of dissolving a nitrateor acetate of Ni and carbonates, oxalates, hydroxides and oxides of Cr,Mn and Zn in an aqueous solution of nitric acid and/or acetic acid.However, the present invention is not limited to them except the Nisalts. Any salts of Cr, Mn and Zn may be used, if they leave no anion inan aqueous solution of nitric acid or acetic acid when they aredissolved in it.

As to the method of applying said treating solution with additive any ofmethods adopted when applying the abovementioned treating solution, suchas the spraying method, roller-coating method and dipping method, may belikewise used. If necessary, also a surface active agent may be furtheradded.

Also the heating can be carried out under exactly the same conditionsand in the same manner, that is, in a temperature range of 200 to 750 C.in a furnace of a non-oxidative gas atmosphere in which, for example,the H content is 2 to 20% and the rest is mostly N However, the thusobtained film is higher in the anticorrosiveness than in the case of theabove mentioned treating solution composed of the Ni salt only.

Further, as a result of making further investigations the presentinventors have discovered that when an Al salt is further added to theabove-mentioned treating solution with additive a film having moreexcellent anticorrosiveness and workability can be obtained.

For example, when a treating solution prepared by adding an Al salt tothe above-mentioned treating solution with additive, which contains oneor more of Cr, Mn and Zn salts in addition to Ni, is subjected to thethermodecomposition in a temperature range of from 200 to 750 C. in afurnace of a nonoxidative gas atmosphere, in which the H content is 2 to20%, there can be formed on the surface of a steel plate a very thinfilm which has been proved to be all the more improved No. 13 in thesame table was prepared by subjecting a steel to a cold-rolling with theabove-mentioned continuous strip rolling apparatus, thereupon toalkali-defatting and pickling and then to an annealing in theabove-mentioned gas atmosphere (this referential steel shall be called anontreated steel plate hereinafter for the convenience of theexplanation).

In the rusting test by an indoor exposure the nontreated steel platerusted Within one month, while samples Nos. 1, 2 and 3 did not rust in 3to 5 months in the same indoor exposure. Further, in the corrosion test,in which samples were dipped in a carbonic acid beverage after beingcoated with an epoxy resin series paint, samples Nos. 1, 2 and 3 showedresults much superior to the nontreated sample in the anticorrosiveness,but somewhat inferior in the paint adhesiveness.

PERFORMANCE TEST COMPARISON TABLE Performance tests (2) Corrosionresistance (1) Antibelow the (8) Paintrusting coating adhesive- SampleNo. Treating solution composition Treating process property film ness 1Nickel nitrate (60 g./l.) Dip-coating 5.0 3 4 2 Nickel acetate (30g./l.) Heating atmos- 4. 3 4

phere (H2:6%, Nzzrest). 3 Nickel nitrate (10 g./l.) plus nickel acetate(10 g./l.) Hiatingigggngera- 2. 5 2 4 ure, Nickel nitrate (20 g.ll.)plus chromium acetate g./i.) Roller-coating. 7.0 4 5 Nickel acetateg./l.) plus manganese nitrate (10 g./l.) Heating atmos- 6.0 3 4 phere(H2:10%, N izrest). 6 Nickel nitrate (20 g./l.) plus zinc nitrate (10g./l.) Heatingotenpera- 4. 5 3 4 ure 7 Nickel acetate (20 g./l.) pluschromium acetate (5 g./l.) 9.5 5 5 plus manganese nitrate (5 g./l.). 8Nickel nitrate (20 g./l.) plus manganese nitrate (5 g./l.) 8.5 4 3 pluszinc nitrate (5 g./l.). Nickel nitrate (20 g./l.) plus aluminium nitrate(5 g./l.) Dip-coating 11.5 5 5 plus chromium acetate (5 g. 10 Nickelnitrate (20 g./l.) plus aluminium nitrate (5 g./l.) Heating atmos- 10.05 5 plus zinc nitrate (5 g./l.). plhere (;Hz:6%,

zzres l1 Nickel acetate (20 gJl.) plus aluminium nitrate (5 g./1.)Heating tempera- 9.0 5 5 plus zinc nitrate (5 g./l.). ture 250 C. 12Nickel acetate (10 g./1.) plus aluminium nitrate (5 g.li.) 12.5 5 5pusflchromium nitrate (5 g./l.) plus manganese nitrate g. 13 (Nontreatedsteel plate) 0.5 1 5 in the anticorrosiveness and workability, as areshown Example 2 in the following examples.

As will be understood from the foregoing, the method of the presentinvention can provide cheaply a steel plate having a strong inactivefilm, just like the surface of a stainless steel, which is particularlysuitable as a material for manufacturing cans therefrom because of beinghigh in the anticorrosiveness and excellent in the paint adhesiveness.

Examples shall be detailed in the following.

Example 1 A cold-rolled steel plate of a thickness of 0.26 mm., whichhad been cold-Worked by a well 'known method, that is, by using acontinuous strip rolling apparatus, but had not yet been annealed, wassubjected to a well known pretreatment, for example, such surfaceadjustments as alkali-defatting and sulfuric acid-pickling, was thendipped in an aqueous solution of nickel nitrate or nickel acetate sothat nickel salt might be deposited on the surface, was immediatelythereupon heated at a temperature of 600 C. in an annealing gasatmosphere of an H content of 6% and the rest being N so as to effect astrain-removing annealing and at the same time to form a nickel film andwas then rolled for refining at a rate of reduction of 1%. Thecomparison of the performances was made on the antirusting property, thecorrosion resistance below the coating film and the paint adhesiveness.The samples Nos. 1, 2 and 3 in the below-mentioned performance testcomparison table are steel plates having a film composed of metallicnickel prepared by the above-mentioned method respectively. On the otherhand, a referential sample The same cold-rolled steel plate as inExample 1 was defatted with an alakli and pickled with sulfuric acid,was then coated with a treating solution prepared by adding one or moreof chromium acetate, manganese nitrate and zinc nitrate into an aqueoussolution of nickel acetate or nickel nitrate by using a roller and wasthen immediately heated at a temperature of 600 C. in a brightlyannealing gas atmosphere of an H content of 10%, the rest being N toform a film. The samples Nos. 4 to 8 in the above-mentioned performancetest comparison table were steel plates prepared by the above-mentionedprocess. It is obviously seen that the film optained by applying atreating solution containing one or more of Cr, Mn and Zn salts inaddition to an Ni salt has performances more excellent than of the filmby Example 1. That is, they showed no rusting in 4 to 10 months in therusting test by indoor exposure and were very high in the corrosionresistance below the coating film in a carbonic acid beverage afterbeing coated with a lacquer.

Example 3 A cold-rolled esteel plate of a thickness of 0.26 mm. whichhad been cold-worked by a well known method, for example, by using acontinuous strip rolling apparatus, thereafter alkali-defatted, sulfuricacid-pickled and then annealed in a reductive atmosphere was once moresurface-treated with alkali-defatting and sulfuric acid-pickling, wasthen dip-coated with a treating solution prepared by adding one or moreof chromium acetate, manganese nitrate and zinc nitrate into an aqueoussolution of a mixture of nickel nitrate and aluminium nitrate and wasthen immediately heated at a temperature of 250 C. in a heated gasatmosphere of an H content of 6%, the rest being N to form a film.

The samples Nos. 9 to 12 in the above-mentioned performance testcomparison table which correspond to this example, did not rust in 9 to13 months in a rusting test by indoor exposure, were not corroded at allbelow the lacquer-coated film in a carbonic acid beverage after beingcoated with a lacquer and were high also in the paint adhesiveness.

In all the Examples 1, 2 and 3 the heating time was very short,amounting to several seconds, though there was a slight differenceaccording to the temperature in the range of 200 to 750 C.

The numerals in the above-mentioned performance test comparison tableare defined as follows:

(1) Antirusting property The period until a rust recognizable byobservation with the naked eye was generated on the surface of the steelplate by the indoor exposure test was represented by the number ofmonths.

(2) Corrosion resistance below the coating film The steel plate wascoated with an epoxy resin series lacquer, had then a scratch of a widthof 0.1 mm. made on it and was dipped for one month in a cola seriescarbonic acid beverage kept at such fixed temperature as 38 C. in theexample and then the degree of corrosion below the coating film in thescratched part was observed. The numerals in the table were defined asfollows by dividing the evaluations into five grades:

-When no coating film peeling was recognized at all.

4-When a coating film peeling of about 0.1 to 0.2 mm.

was recognized.

3-When a coating film peeling of about 0.2 to 0.4 mm.

was recognized.

2When a coating film peeling of about 0.4 to 0.8 mm.

was recognized.

1When a coating film peeling of about 0.8 to 2.0 mm.

was recognized.

(3) Paint adhesiveness A lacquer was applied and was then bonded with abinder and then the tensile strength was measured and was evaluated bymaking the maximum value 5 and the minimum value 1 as a scale of thelacquer adhesivness.

What is claimed is:

1. A method of imparting anti-corrosive properties to surface-treatedsteel plates comprising applying to the surface of a steel plate anaqueous solution consisting essentially of at least one compoundselected from the group consisting of nitrates and acetates of Cr, Mnand Zn in admixture with at least one compound selected from the groupconsisting of a nitrate and acetate of Ni to coat the surface thereofsaid steel plate having been previously subjected to a surface-cleaningpretreatment and then heating the thus-coated steel plate in anonoxidative gas atmosphere to cause a thermodecomposing reaction toform a film containing metallic nickel and at least one oxide selectedfrom the group consisting of oxides of Cr, Mn and Zn.

2. A method of imparting anti-corrosive properties to surface-treatedsteel plates comprising applying to the surface of a steel sheet anaqueous solution consisting essentially of at least one compoundselected from the group consisting of nitrates and acetates of Cr, Mnand Zn in admixture with at least one compound selected from the groupconsisting of a nitrate and acetate of Ni and further containing atleast one compound selected from the group consisting of a nitrate andacetate of Al to coat the surface thereof, said steel plate having beenpreviously subjected to a surface-cleaning pretreatment and then heatingthe thus-coated steel plate in a nonoxidative gas atmosphere to cause athermodecomposing reaction to 8 form a film containing metallic nickel,an oxide of aluminum and at least one oxide selected from the groupconsisting of oxides of Cr, Mn, and Zn.

3. A method of imparting anti-corrosive properties to surface-treatedsteel plates comprising applying to the surface of a steel plate anaqueous solution consisting essentially of at least one compoundselected from the group consisting of nitrates and acetates of Cr, Mnand Zn in admixture with at least one compound selected from the groupconsisting of a nitrate and acetate of Ni containing 0.5 to g./l. of Niions whereby less than 20 g./l. of Cr+++, less than 40 g./l. of Zn ionsand less than 20 g./l. of Mn ions are present in the solution andfurther containing at least one compound selected from the groupconsisting of a nitrate and acetate of Al so that the content of Al ionsis less than 20 g./l. to coat the surface of the steel plate, said steelplate having been previously subjected to a surface-cleaningpretreatment and then heating the thus coated steel plate in anonoxidative gas atmosphere to cause a thermodecomposing reaction toform a film containing metallic nickel, an oxide of aluminum and atleast one oxide selected from the group consisting of oxides of Cr, Mn,and Zn.

4. A method of imparting anti-corrosive properties to surface-treatedsteel plates comprising applying to the surface of a steel plate anaqueous solution consisting essentially of at least one compoundselected from the group consisting of nitrates and acetates of Cr, Mnand Zn in admixture with at least one compound selected from the groupconsisting of a nitrate and acetate of Ni containing 1 to 20 g./l. of Niions whereby less than 10 g./l. of Cr+++, less than 10 g./l. of Zn ionsand less than 10 g./l. of Mn ions are present in the solution andfurther containing at least one compound selected from the groupconsisting of a nitrate and acetate of Al so that the content of Al ionsmay be less than 10 g./l. to coat the surface of the steel plate, saidsteel plate having been previously subjected to a surface-cleaningpretreatment and then heating the thus coated steel sheet in anonoxidative gas atmosphere to cause a thermodecomposing reaction toform a film containing metallic nickel, an oxide of aluminum and atleast one oxide selected from the group consisting of oxides of Cr, Mn,and Zn.

5. A method of imparting anti-corrosive properties to surface treatedsteel plates according to claim 1 wherein the pH of said treatingaqueous solution is 3 to 4.

6. A method of imparting anti-corrosive properties to surface treatedsteel plates according to claim 1 wherein the thickness of the treatedfilm is less than la.

7. A method of imparting anti-corrosive properties to surface-treatedsteel plates according to claim 1 wherein the thickness of the treatedfilm is less than 1 and an organic coating film is formed directly onsaid film and is heated and dried.

8. A method of imparting anti-corrosive properties to surface treatedsteel plates according to claim 1, wherein the pH of said treatingaqueous solution is 3 to 4 and wherein the thickness of the film formedby the coating treatment is less than 1;.

9. A method of imparting anti-corrosive properties to surface treatedsteel plates according to claim 2, wherein the pH of said treatingaqueous solution is 3 to 4 and wherein the thickness of the film formedby the coating treatment is less than i 10. A method of impartinganti-corrosive properties to surface treated steel plates according toclaim 3, where in the pH of said treating aqueous solution is 3 to 4 andwherein the thickness of the film formed by the coating treatment isless than I 11. A method of imparting anti-corrosive properties tosurface treated steel plates according to claim 4, wherein the pH ofsaid treating aqueous solution is 3 to 4 and wherein the thickness ofthe film formed by the coating treatment is less than 1;.

12. A method of imparting anti-corrosive properties to surface-treatedsteel plates according to claim 1 wherein the steel plate is acold-rolled steel sheet and the coated cold-rolled steel sheet is heatedat 200 to 750 C. in a nonoxidative gas atmosphere composed of 2 to 20% Hand the rest being nitrogen gas to form the film on the surface of thesheet and to remove strains in the steel sheet.

13. A method of imparting anti-corrosive properties to surface-treatedsteel plates according to claim 2 wherein the steel plate is acold-rolled steel sheet and the coated cold-rolled steel sheet is heatedat 200 to 750 C. in a nonoxidative gas atmosphere composed of 2 to 20% Hand the rest being nitrogen gas to form the film on the surface of thesheet and to remove strains in the steel sheet.

14. A method of imparting anti-corrosive properties to surface-treatedsteel plates according to claim 3 wherein the steel plate is acold-rolled steel sheet and the coated cold-rolled steel sheet is heatedat 200 to 750 C. in a non-oxidative gas atmosphere composed of 2 to 20%H and the rest being nitrogen gas to form the film on the surface of thesheet and to remove strains in the steel sheet.

15. A method of imparting anti-corrosive properties to surface-treatedsteel plates according to claim 4 wherein the steel plate is acold-rolled steel sheet and the coated cold-rolled steel sheet is heatedat 200 to 750 C. in a nonoxidative gas atmosphere composed of 2 to 20% Hand the rest being nitrogen gas to form the film on the surface of thesheet and to remove strains in the steel sheet.

References Cited UNITED STATES PATENTS 3,436,511 4/1969 lRath 106-1 X3,573,992 4/1971 Grubb et al 117-130 R X 3,579,383 5/1971 Turner 117-130R X 3,522,108 7/1970 Yamamoto et al. 117-70 C X 2,101,950 12/1937McGOhan 117-130 R X 3,069,765 12/ 1962 Simpelaar 117-130 R X 3,223,52312/1965 Adler 117-130 R X 3,468,724 9/1969 Reinhold 117-130 R X3,511,690 5/1970 Aharoni 117-71 M 3,544,434 12/1970 Giller et a1. 117-70C X FOREIGN PATENTS 562,046 6/1944 Great Britain 117-130 R 223,4688/1959 Australia 117-75 551,869 1/1958 Canada 117-71 M ALFRED L.LEAVITT, Primary Examiner I. R. BATTEN, JR., Assistant Examiner US. Cl.X.R.

